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Found one definition

1.                 From The Free On-line Dictionary of Computing (19 January 2023):
                   
   
   
                   
                     
   maximum segment size
   MSS
      <networking> (MSS) The maximum amount of TCP data that a
      node can send in one segment.  This should be the size of
      the receiver's reassembly buffer to try to avoid
      fragmentation.
      The equivalent at the physical layer is "Maximum
    Transmission Unit".
      (1998-03-06)